1. Field of the Invention
The present invention relates to a technique for secure communications on FM radio channels and, more particularly, to a technique which superimposes on an FM signal a masking signal of the same bandwidth.
2. Description of the Prior Art
Mobile radio and cordless telephone are among the communication services which have stimulated a demand for secure radio communications. To provide the degree of security required for these services digital ciphers can be built, as disclosed in, for example, U.S. Pat. No. 4,126,761 issued to D. Graupe et al. on Nov. 21, 1978 where the analog signal is converted to a digital signal and then encoded and converted back to an analog signal for transmission. However, a digitized analog signal requires more bandwidth for expansion than the same nondigitized analog signal. Some secret communications systems use bandwidth expansion means to mask signals as disclosed in, for example, U.S. Pat. Nos. 3,638,121 issued to J. J. Spilker, Jr. on Jan. 25, 1972 and 4,179,658 issued to D. R. Bitzer on Dec. 18, 1979. For frequency modulated (FM) radio services, frequency bandwidth is at a premium.
A means for masking an analog signal by the linear addition of a noiselike waveform generated from the original signal is disclosed in U.S. Pat. No. 4,361,729 issued to L. A. Barnes, Jr. et al. on Nov. 30, 1982.
The problem, therefore, remaining in the prior art is to provide secure communications for analog signals and in particular FM signals without bandwidth expansion.